1. Field of the Invention
The present invention relates to a hydraulic control device of an automatic transmission, and specifically to hydraulic technologies for a hydraulic control device of an electronically-controlled automatic transmission capable of electronically controlling an engaging pressure of each of engaging elements or controlling elements such as clutches and brake bands directly by means of a transmission ECU (electronic control unit), and of providing simplified hydraulic circuits, reduced hydraulic component parts, small-sized valve bodies, and expanded design flexibility, and lighter hydraulic packages.
2. Description of the Prior Art
In recent years, there have been proposed and developed various electronically-controlled automatic transmissions of simplified hydraulic circuits, reduced hydraulic component parts, and small- sized valve bodies. One such electronically-controlled transmission (which will be hereinafter abbreviated to an xe2x80x9cECT transmissionxe2x80x9d) has been disclosed in Japanese Patent Provisional Publication No. 8-121586. The ECT transmission disclosed in the Japanese Patent Provisional Publication No. 8-121586, has a low-and-reverse brake (LandR/B) pressure control device through which a low-and-reverse brake (LandR/B) is applied or engaged in a drive range and first gear, and released or disengaged in a drive range and either of second, third and fourth (overdrive) gears. In more detail, as shown in FIG. 12, the above-mentioned low-and-reverse brake (LandR/B) pressure control device uses two different hydraulic pressures, namely one being a second brake pressure P2ND which is applied to a second brake for the purpose of applying the second brake in a drive range and second gear or in a drive range (D range) and fourth gear (OD gear), and the other being an overdrive clutch pressure POD which is applied to an overdrive clutch for the purpose of engaging the overdrive clutch in a drive range and third gear or in a drive range and fourth gear. The low-and-reverse brake (LandR/B) pressure control device functions to forcibly drain the pressure supply line of the low-and-reverse brake in the D range and 2nd gear, in the D range and 3rd gear, or in the D range and 4th gear, in which at least one of the second brake pressure P2ND and the overdrive clutch pressure POD are created or produced.
In the hydraulic control device of the electronically-controlled automatic transmission disclosed in the Japanese Patent Provisional Publication No. 8-121586, however, during shifting from first to second gear wherein the low-and-reverse brake (LandR/B) is changed from its applied state to its released state, the pressure supply line of the low-and-reverse brake pressure is forcibly drained by using the second brake pressure P2ND for the second brake. During the shifting from 1st to 2nd gear, on the other hand, the second brake is changed from its released state to its applied state. The prior art hydraulic control device is designed so that, during the shifting from 1st to 2nd gear, a line pressure PL, varying depending on engine speed, acts on one side of a land opposing the other side of the land on which the second brake pressure P2ND is applied. In other words, the line pressure PL serves as an opposing pressure to the second brake pressure P2ND. For the reasons discussed above, it is difficult to precisely control a timing of forcible draining of working oil from the pressure supply line of the low-and-reverse brake by using the line pressure PL as the opposing pressure. That is, the prior art hydraulic control device has the following drawbacks.
(A) When the second brake pressure P2ND lower than a maximum hydraulic pressure level during shifting is used as a valve-position switching pressure, there is an increased tendency for the low-and-reverse brake (LandR/B) pressure to be forcibly released before termination of the shifting from 1st to 2nd gear. This exerts a bad influence upon the automatic shifting control. Actually, during the shifting operation from first to second gear, a delicate control for pressure-release from the low-and-reverse brake and a delicate control for pressure-application to the second brake have to cooperate with each other so as to realize a shifting operation without any shift shock. If the low-and-reverse pressure drops down to essentially atmospheric pressure by way of the previously-noted forcible draining before termination of the shifting from 1st to 2nd gear, a lack of total engaging capacity in both the low-and-reverse brake and the second brake may occur, and thus results in an undesirable rise in engine speed. This acts as a new factor in the causation of shift shock.
(B) When the second brake pressure P2ND equal to the maximum hydraulic pressure level during shifting is used as a valve-position switching pressure, there is an increased tendency for the low-and-reverse brake (LandR/B) pressure to be forcibly released after the application or engagement of the second brake has been completed. Therefore, assuming that the low-and-reverse brake (LandR/B) pressure is unexpectedly kept at a high level owing to a system failure, during a particular time period from a time when the shifting operation ends to a time when the previously-noted forcible draining starts, the automatic transmission will fall into a so-called interlocking state in which the low-and-reverse brake and the second brake are both applied or engaged.
Accordingly, it is an object of the invention to provide a hydraulic control device of an automatic transmission, which avoids the aforementioned disadvantages of the prior art.
It is another object of the invention to provide a hydraulic control device of an electronically-controlled automatic transmission, which has a fail-safe function capable of providing an optimal timing of a forcible pressure-release of an engaging-element pressure (simply, an engaging pressure) applied to an engaging element or an optimal timing of a forcible oil drain from the engaging element, without exerting a bad influence upon an automatic shifting control and without providing an undesired automatic transmission interlock in presence of a system failure.
In order to accomplish the aforementioned and other objects of the present invention, a hydraulic control device of an automatic transmission having a first engaging element engageable or disengageable by a first engaging-element pressure regulated during shifting, a solenoid valve creating a solenoid pressure in response to a solenoid drive signal, and a pressure regulator valve creating the first engaging-element pressure applied to the first engaging element by using the solenoid pressure and a regulated line pressure thereof as operating signal pressures, comprises a fail-safe valve adapted to forcibly drain the first engaging-element pressure from the first engaging element by using a second engaging-element pressure applied to a second engaging element brought into an engaging state from a releasing state during a shifting operation during which the first engaging element is changed from an engaged state to a disengaged state, the fail-safe valve including a spool and using the second engaging-element pressure acting on the spool in one axial direction and an opposing pressure acting on the spool in the opposing direction as operating signal pressures, and the fail-safe valve switching to a drain position when the second engaging-element pressure is regulated toward a specified fail-safe valve operating point pressure higher than a maximum pressure value of the second engaging-element pressure regulated during the shifting operation and lower than a maximum possible engaging-element pressure.
It is more preferable that the fail-safe valve uses a third engaging-element pressure applied to a third engaging element already kept in an engaged state in addition to the second engaging-element pressure to forcibly drain the first engaging-element pressure during the shifting operation, and the fail-safe valve has at least two valve construction including at least a first fail-safe valve having a spool operable by the second engaging-element pressure and the opposing pressure, both serving as operating signal pressures for the first fail-safe valve, and a second fail-safe valve having a spool operable by the third engaging-element-pressure and the opposing pressure, both serving as operating signal pressures for the second fail-safe valve, and the first and second fail-safe valves are operable independently of each other. Preferably, the fail-safe valve may be disposed upstream of the pressure regulator valve, and thus the fail-safe valve can operate to forcibly drain an input pressure line for the pressure regulator valve during a fail-safe operating mode. More preferably, a fail-safe pressure valve is provided to create the opposing pressure acting on the spool of the fail-safe valve in the opposing direction, and the fail-safe pressure valve has a spool having the same pressure-intensified ratio as the pressure regulator valve and uses a line pressure as an input pressure, and the spool of the fail-safe pressure valve receives at one end a pilot pressure acting in one axial direction and receives at another end an output pressure acting in the opposite direction.